Compartnentalisation methods based on water-in-oil emulsions have recently been developed for use in repertoire selection methods (Tawfik & Griffiths 1998, Ghadessy et al 2001). Compartmentalisation segregates individual genes and their encoded products (delivered either via cells (Ghadessy et al, 2001) or expressed in situ (Tawfik & Griffiths, 1998)) into discrete, physically separate aqueous compartments, thus ensuring the linkage of genotype and phenotype during the selection process.
WO99/02671. (which is incorporated herein by reference) describes a method for isolating one or more genetic elements encoding a gene product having a desired activity. Genetic elements are first compartmentalised into microcapsules, which are preferably formed by emulsification, and are then transcribed and/or translated to produce their respective gene products (RNA or protein) within the microcapsules. Alternatively, the genetic elements may be contained within microcapsules of the emulsion and transcription and/or translation (expression) of the gene product can take place within using the cellular machinery. Genetic elements that produce a gene product having a desired activity can be subsequently sorted. For example, in some cases sorting may be due to the desired activity inducing a change in the microcapsule. In other cases sorting may be due to the desired activity inducing a change in the genetic element.
The method disclosed in WO99/02671 works well with bacteria. Although the cellular subcompartmentalisation approach could in principle be extended to include eukaryotic cells, e.g. yeast, insect or mammalian cells, for some applications it would be desirable to provide in situ expression directly in microcapsules using an in vitro eukaryotic transcription/translation system.
Previous successful expression of a prokaryotic enzyme, Hae methylase, has been reported using bacterial S30 extracts in emulsion (Tawfik D. & Griffiths A. D. 1998). However, such methods are not suitable for some proteins of interest, for example large multi-domain proteins and ribonucleoproteins which frequently cannot be expressed in functional form using bacterial extracts.
Thus it can be seen that a method which enables in situ expression directly in microcapsules using an in vitro eukaryotic transcription/translation system would provide a contribution to the art.